You need to go to the person's profile and block them. If they have Timeline enabled, click on the cog-wheel on the right and select Report/Block. If they still have the old profile style, it'll be in the left-most column under their list of friends.

In addition, your son should go to his Subscription management page and change the settings so that either only friends or friends of friends could comment on his public updates.

Moreover, he should not post public updates if he's concerned with strangers posting inappropriate comments. This is accomplished at the time of publishing just one update by changing privacy to Friends (Facebook remembers the last privacy setting for posts).